Optical current transformers have been known for a long time. Generally, a current transformer is understood to be a measuring transducer having a sensor head for potential-free measurement of alternating currents. Optical current transformers make use of the Faraday effect for measurement. The Faraday effect describes the rotation of the polarization of a linearly polarized electromagnetic wave upon passing through a transparent medium to which a magnetic field that is constant over time is applied parallel to the direction of propagation of the wave.
Optical current transformers are increasingly being used at high-voltage potential as well. Such an optical current transformer is known from DE 198 02 191 B4, for example.
The Faraday effect is generally temperature-dependent since the rotation of the polarized wave is dependent on the material properties of the transparent medium, and said properties change with temperature. If the temperature dependence is not taken into account during the evaluation of the measurement, then this can lead to measurement errors in the case of the measurement values determined for the altering current. In order to compensate for such measurement errors, an additional temperature measurement at high-voltage potential is carried out according to the prior art. In this case, the measurement of the temperature can be carried out both electronically and optically.
Optical temperature sensors are for example sensors on the basis of fiber Bragg gratings. Sensors having semiconductor elements which function as temperature-dependent optical band-edge filters or sensors which utilize the temperature-dependent decay time of the fluorescence of crystals are also known. The disadvantage of optical temperature sensors resides in their higher complexity in comparison with electronic sensors.
According to the prior art, electronic temperature sensors are digital sensors which are implemented by means of microprocessors. Such temperature sensors based on digital signal processing require a high operating voltage in the range of a few volts.
Moreover, they have to be continuously supplied with enough electrical energy.
In order to cover the demand for electrical energy, sensors which are supplied with energy by means of an optical waveguide in order to fulfill the measurement task are known in the case of digital measurement of the temperature at high-voltage potential. In this case, light of a powerful laser is guided from a ground station by means of the optical waveguide to the optical current transformer. Powerful lasers having a power in the range of 100 mW to 500 mW are typically used in order to provide enough energy for the digital temperature measurement. Within the current transformer or the temperature sensor there is an arrangement consisting of a plurality of photoreceivers which convert the light of the laser into electrical energy for the operation of the digital temperature sensor. After the measurement task has been fulfilled, the measurement signal is passed back to the ground station via a further optical waveguide. On account of the great high optical power of the lasers used, a sufficient operating safety must always be ensured.